A. Field of invention
The present invention relates generally to methods and apparatus for fluid dispensing, and more particularly to a new and improved method of dispensing fluids consisting of solutes dissolved or particles suspended in a volatile fluid dispersion medium and the apparatus used for employing such method.
B. Description of Related art
The present invention relates to methods and systems for dispensing fluids from a nozzle, meaning an orifice of limited size, wherein the nature of the fluid being dispensed is such that evaporation of the fluid dispersion medium may cause solid matter to be deposited at the nozzle, leading to clogging. An example of such a system is an ink jet printing system. In its simplest form, a fluid dispensing system, such as an ink jet printing system, will include a flow control means, such as a control valve, and a nozzle connected to the flow control means by a fluid conduit. It is frequently desirable to employ a nozzle that is of a relatively small bore and often one that is of a smaller bore than the fluid conduit. Increased print resolution is achieved, in part, by reduction in volume of ink dispensed in an operational cycle of an ink jet printing system, frequently requiring a reduction in the size of the nozzle bore. The limits of print legibility requires systems used in the field of ink jet printing to perform precisely and at high frequencies with uniformly replicable results. In addition, the ink dispensed by an ink jet printing system generally consists of a fluid dispersion medium with dissolved solute or suspended particles, generally solid pigmentation. It has been found that when the fluid dispersion medium of the ink is volatile, ink jet printing systems are often subject to clogging of the nozzle. The method and apparatus of the present invention were developed in response to the specific problems of ink jet printing system failures, and for that reason, ink jet printing systems are used as the model for description of the method and apparatus of the present invention which is intended to prevent the problem of clogging nozzles in such systems. Use of the ink jet printing model is not intended to limit the applicability of the method to that field, and it is anticipated that the invention can be successfully utilized in other circumstances. For the sake of brevity, the term solvent is used to refer to any fluid dispersion medium whether a solvent of a solution or the fluid base of a suspension, as the invention is applicable in both cases.
In the process of ink jet printing, it is necessary to accurately control the deposition of ink on the object over a distance which requires a mechanism to send the ink at a high velocity in order to travel to the print material. In many cases multiple nozzles are used in an array and the degree of resolution that can be achieved by a printing system depends upon the number and size of the nozzles as well as the density of the nozzle array. It is frequently desirable to pack the nozzles in a tight array with each of the nozzles in the array being connected to a control valve that is offset from the nozzle array. Since the control valves are frequently larger than the nozzles, the separation of valves from the nozzle array allows dense nozzle packing. In addition, the offset of the valves from the nozzle array provides added flexibility and convenience in construction and maintenance. The distance of the offset of the valves from the nozzle array is a design variable determined by the particular physical characteristics of the system components. A section of flexible tubing is commonly used as the fluid conduit to complete the connection between each control valve and the respective nozzle controlled by it. For ordinary engineering concerns, the obvious choice for the material of such tubing is a material that is impervious to the fluid being handled.
The operational cycle of a simple unit in such systems involves the opening of the control valve which allows the ink source pressure to expel ink from the nozzle, followed by the closing of the control valve which isolates the nozzle from the source pressure and the flow of ink stops. The system units are operated at high frequencies such that ink is dispensed one drop at a time. Preferably the outer end of the nozzle is left open to the atmosphere to allow its use at any time, and when the control valve is closed, the ink not expelled is retained in the nozzle by surface tension.
The ink used in ink jet printing generally comprises pigmented solids or solutes suspended or dissolved in a solvent base. Solvents of relatively low volatility such as water or oil are commonly used in ink jet printing on porous, absorbent materials such as cardboard, the solvent being rapidly absorbed into the substrate to present a dry surface in a relatively short time. However, for ink jet printing on nonporous media, such as plastic, metal or coated cardboard surfaces, more highly volatile solvent is necessary to avoid running and smudging. Methyl Ethyl Ketone ("MEK") is a volatile solvent frequently used for printing on non-absorbent, smooth surfaces and has the advantage of being able to wet many surfaces that will not accept other solvents. A disadvantage to the use of volatile solvents such as M.E.K. is a frequently encountered problem of nozzles that become clogged when not in use for a relatively short period of time. When the system is not in use, the evaporation of the solvent through the open end of the nozzle results in a scab that can thicken into a plug that clogs the nozzle. While evaporation caused clogging may occur over sufficient time with any solvent that is subject to evaporation, it has been found that an ink jet printing system that performs acceptably with less volatile solvent based inks, becomes clogged when a MEK based ink is used and the system is turned off in the normal course of operation. It is believed that the formation of the solid plug occurs in a conventional system using impervious tubing because all evaporation of the ink solvent occurs across the surface of the meniscus at the nozzle orifice and that the speed of the withdrawal of the meniscus from the nozzle orifice is slow enough that the meniscus gradually stops, and the interface gradually thickens as ink is drawn to the interface and ink solids are continuously left at the interface by the evaporation of the ink solvent. This understanding of the process of plug formation in prior art systems is illustrated in FIG. 1. which shows a conventional tube and nozzle arrangement. In FIG. 1, the tubing is identified by the numeral 11, the nozzle is identified by the numeral 13, and the ink within the tube 11 is identified by the numeral 31, with the successive ink-air interfaces identified by numerals 1 through 6 respectively. As illustrated in the Drawings, the inside diameter of the nozzle is typically smaller than that of the tubing and is therefore more prone to clogging. In such conventional systems, it is believed that, over time, the ink solvent is drawn out through the interface, and all or much of the ink solids will be deposited at or in the nozzle.
Operating a conventional ink jet printing system with volatile solvents may require preventative routine maintenance or a specific operational sequence including a start-up sequence or a purging sequence to eliminate the plug at the nozzle. Other attempted solutions include covering or continuously wetting the orifices when inactive. Such adaptations involve extra steps not normally part of the ink jet printing process and are time consuming and not particularly effective. Another alternative is to attempt to resist nozzle clogging by reducing the volume of ink in the fluid conduit by shortening the fluid conduit or by integrating the nozzle into the valve to eliminate the fluid conduit. Elimination of the conduit between the nozzle and valve requires the construction of a preassembled manifold with the highest nozzle density possible, often slanting the valve-nozzle assembly to achieve greater density. The purpose of such an arrangement is to minimize the passage to the nozzle orifice to minimize the volume of ink subject to evaporation to reduce the size of the pigment scab to a size that will not plug the nozzle. Integrated systems are relatively costly to construct and the individual components are not easily, if at all, replaced or serviced. In addition a size limitation is imposed by the inability to offset the valves from the nozzle array. Simply shortening the fluid conduit does not eliminate the design limitations. As stated above, the reduction of the fluid conduit places limitations on the minimum proximity of the nozzles since the valve size determines the spacing of the smaller nozzles. Therefore, print resolution is expected to suffer unless the valves and nozzles are separated by a substantial length of fluid conduit.